Just ten years ago only 11% of the cannabis on UK streets was grown within its shores.

But now that figure stands at 60%, according to a report produced by the charity DrugScope.

Network of farms

In London, where the class C drug is worth £100m a year, the Metropolitan Police says it has closed down two to three factories a day and destroyed 1.5 tonnes of cannabis a week over the past 18 months.

Around 1,500 cannabis farms were discovered in the capital alone during the last two years, compared with just 500 in the previous two years.

But contrary to the common perception of benign hippies growing marijuana in their lofts, the vast bulk of the industry is in the grip of criminals.

According to the report, 75% of Britain's cannabis factories are run by Vietnamese gangs.

Harry Shapiro of DrugScope says it is believed that Vietnamese gangsters in Canada initially turned their attentions towards manufacturing cannabis after losing control of the illegal timber trade.

Their counterparts in the UK were able to draw on their Canadian contacts' expertise and set up their own network of farms, often in ordinary-looking homes.

'Slave labour'

Often they are maintained by illegally-trafficked young men, Mr Sharpiro says, living in tiny cupboards while they repaid their debts to the gangs.

"It's slave labour - these kids will be sleeping on a grubby mattress under hot, bright lights," he says.

"It's a pretty miserable existence for them. This is a million miles away from grannies growing their own."

Most of the farms are in London, he added, but others have been raided as far afield as Glasgow, Birmingham, East Anglia, south Wales, the North East and Yorkshire.

During a recent raid on an address in Lewisham, officers discovered approximately 1,000 cannabis plants

Commander Allan GibsonMetropolitan police

Despite a widespread impression to the contrary, most of the marijuana on the streets is not getting stronger because the more potent skunk strains are more expensive to cultivate.

A slump in the Moroccan cannabis crop by 62% thanks to bad weather and a government clampdown has also made it more attractive for criminals to produce the drug domestically.

But despite the surge in cannabis produced in the UK, Mr Shapiro says a clampdown by police has stopped prices from falling.

"In fact, officers have been so effective in some parts of the country that there has actually been a cannabis drought," he adds.

Commander Allan Gibson, the Metropolitan Police Service lead on drugs, says his force is closing down factories "on a daily basis".

He adds: "Typically when officers carry out raids on cannabis factories we discover that the entire house is being used for the cultivation of cannabis, with hundreds of plants being grown in most rooms.

"During a recent raid on an address in Lewisham, officers discovered approximately 1,000 cannabis plants and associated hydroponics equipment used for cultivating cannabis."

'Fiddle their meters'

He said thermal imaging technology was often used to detect cannabis factories, as they release up to 10 times as much heat as an average home.

But Martin Duffell of British Gas told BBC Five Live that the gangs rarely paid for their electricity, bypassing their meters and linking up their power supply to street lights.

"If they were using huge amounts of electricity to power their lights and other equipment their bills would shoot up," he said.